Apparatus and method for fabricating high purity, high density metal matrix composite materials and the product thereof

ABSTRACT

A method of production of large Ingots of neutron attenuating composites using a vacuum-bellows system allows for large cross-sectional shapes to be extruded and rolled. This method uses a vacuum-bellows technology which allows the manufacturing of large 8–16 inch diameter ingots (50–450 lbs. each). A variety of primary metal matrix materials can be used in this technology. High specific strength and stiffness can be achieved because the technology allows for final densities of 99% and higher. The vacuum-bellows technology allows metals and ceramics to blend and mesh together at compression pressures of 800 tons with elevated temperatures. The controlled compression movement allows for any oxide layer, on the metal, to be broken up and consolidated with the chosen ceramic particulate. One application is to blend boron-rich ceramics and high purity (99.5–99.99%) aluminum particulates together and produce a large ingot using this vacuum-bellows technology. The vacuum-bellows technology allows the ingot to be extruded to large cross-sectional sizes that some applications need (9–12 width). By controlling the amount of boron-rich ceramics, by volume or weight, certain B-10 isotope areal densities can be accomplished. These B-10 isotopes attenuate neutrons in nuclear fuel. Other elements, which have high, cross-sectional Barn values can be used. These are, but not limited to, samarium and gadolinium oxides, nitrides, carbides and silicides.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to the field of metal matrixcomposite materials including those materials which have high,cross-sectional Barn values including ceramics, and metals including butnot limited to boron, samarium and gadolinium oxides, nitrides, carbidesand silicides thereof. These ceramic's and metal additives are used forattenuating neutrons in nuclear fuel. The invention relates morespecifically to an apparatus and method for producing such materialswith high strength, high density and durability and large ingot sizes.

2. Background Art

Most methods for producing metal matrix composites use liquid mixing anddry powder Iso-pressing technology. Liquid mixing is very limited inregard to the size and type of particulates, which can be stirred ormixed into the molten melt. Harmful metal-ceramic reactions, which occurat elevated temperatures during the melt, limited percentages of ceramicparticulate additive and low specific strength and stiffness are allproblems associated with molten stir fabrication of metal matrixcomposites. Iso-pressed metal matrix composites show good specificstrength and stiffness but have a problem with the ingot productionsize. This problem is associated with available Iso-press equipment. Themaximum inner diameter of the largest manufactured high pressureIso-press is approximately 12 inches. Because of consolidation shrinkageand latex bag tooling, the largest ingot diameter that could be producedis approximately 8 inches. Low green density areas are also evident inthe center area of the Iso-pressed 8-inch metal matrix ingots. Thesepresent processing problems and reduce physical strength properties ofthe composite. Because of such size limitations, large cross-sectionalextrusions cannot be made and high extrusion ratios, which are a key toproducing good physical properties, cannot be accomplished.

SUMMARY OF THE INVENTION

The present invention comprises a method of producing large ingots ofboron-rich metal matrix composite materials as well as the apparatusused in that process and the resulting product.

This method uses a vacuum-bellows technology, which allows themanufacturing of large 8–16 inch diameter ingots (50–450 lbs. each). Avariety of primary metal matrix materials can be used in thistechnology. High specific strength and stiffness can be achieved becausethe technology allows for final densities of 99% and higher. Thevacuum-bellows technology allows metals and ceramics to blend and meshtogether at compression pressures of 800 tons with elevatedtemperatures. The controlled compression movement allows for any oxidelayer, on the metal, to be broken up and consolidated with the chosenceramic particulate. One application is to blend boron-rich ceramics andhigh purity (99.5–99.99%) Aluminum particulates together and produce alarge ingot using this vacuum-bellows technology. The vacuum-bellowstechnology allows the ingot to be extruded to large cross-sectionalsizes that some applications need (9–12 width). By controlling theamount of boron-rich ceramics, by volume or weight, certain B-10 isotopeareal densities can be accomplished. These B-10 isotopes attenuateneutrons in nuclear fuel. Other elements which have high,cross-sectional Barn values can be used. These are, but not limited to,samarium and gadolinium oxides, nitrides, carbides and silicides. Otherapplications involve large cross-sectional extrusions which are producedin a variety of different metal-ceramic composites to improve structuralproperties, such as in sailing masts, building structures, robotic arms,automotive and aerospace applications.

It will be seen hereinafter that the novel process disclosed hereinprovides an essentially new metal matrix material exhibiting higherfinal densities and better physical strength properties. This new metalmatrix material is not readily achievable using prior art methods offabrication that require certain chelating additives which reduceachievable densities and physical strength and durability properties.Moreover, the disclosed process makes possible the production of largersize ingots, which are especially advantageous for use in neutronattenuation applications as well as numerous other applications. Apreferred embodiment of the novel process comprises the steps of:

-   -   1. Preparing the powder mixture;    -   2. Compacting the powder mixture;    -   3. Outgassing and sintering the mixture;    -   4. Vacuum pressing the mixture;    -   5. Cooling the resulting billet and machining as desired; and    -   6. Checking billet density and, if necessary repeating steps 2        through 5 as required achieving desired density.

BRIEF DESCRIPTION OF THE DRAWINGS

The aforementioned objects and advantages of the present invention, aswell as additional objects and advantages thereof, will be more fullyunderstood hereinafter as a result of a detailed description of apreferred embodiment when taken in conjunction with the followingdrawings in which:

FIG. 1 is a partially cross-sectioned view of an apparatus which may beemployed in carrying out a preferred embodiment of the inventive processof the invention, wherein the apparatus is shown in its pre-compressionconfiguration; and

FIG. 2 is a view of the apparatus of FIG. 1, but showing the apparatusin its powder compression configuration.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

A preferred apparatus 10 of the invention is shown in FIGS. 1 and 2. Itis uniquely configured to permit vacuum pressing of large billets of theneutron-attenuating metal-ceramic composite material of the presentinvention. Apparatus 10 comprises die 12 having a die punch 14. The dieis contained in an outer housing 16 having a base 20. Controlledcompression of the die punch 14 is obtained by means of a bellows 22having a pressure plate 24 and being secured to the die 12 by a vacuumtight ring seal 18. Die 12 has a plurality of degassing ports 25 whichexit into a chamber 30 formed around the die within outer housing 16. Adegassing outlet pipe 26 is the only exit point for gases within chamber30. A vacuum valve 28 determines whether outlet pipe 26 is open toambient or closed. After a powder mixture has been prepared as will bedisclosed hereinafter, the mixture 15 is loaded into die 12 as shown inFIG. 1. Force is then applied by means of a press (not shown) topressure plate 24 compressing bellows 22 and forcing die punch 14 intodie 12. During the application of pressure to the material 15, thematerial outgasses. The gases are routed through ports 25 into chamber30 and though outlet 26 with vacuum valve 28 in its open state. Thesesteps are repeated after adding more powder mixture to the die until thedie is substantially full and all of the mixture has been compacted andoutgasses to a partial extent.

The die is then placed in a retort of appropriate size and the retort isconnected to a vacuum pump to substantially remove all remaining gases.The valve 28 is then closed and the die is then placed in an oven forsintering at a selected temperature for a selected period of time (i.e.,800° F. for 6 hours). Further evacuation and heating may then berepeated as required to achieve a desired characteristic. Uponcompletion of sintering and oven and further pressure is applied to thesintered material, with vacuum still maintained, i.e. up to 800 tons fora 14 inch diameter billet. The vacuum is then released and the billet isthen allowed to cool before being removed from the die. After removalfrom the die, the billet density is ascertained and, if necessary, theprocess is repeated until the desired density is reached.

An important feature of the apparatus 10 is the provision of anoutgassing path that may selectively be opened or dosed during theprocess of the invention. This feature permits powder compaction beforesintering and vacuum pressing at high temperature without permittingformation of metal oxides in the powder mixture and resulting billet. Byavoiding the formation of metal oxides, such as Al₂O₃, powder compactioncan be very effective and sintering and vacuum pressing can achievedensities over 99% of theoretical without requiring chelating agents ofthe prior art.

In a preferred embodiment, the initial powder mixtures comprises 5–40%wt of boron carbide and 60–95% wt of aluminum, magnesium, titanium andthe like. No chelating additives are used. The initial powder mixturehas a grain size of 3–200 μm achieved by use of a mesh screen to removelarger material. The ceramic and metal matrix powder mixture ispreferably kept at least 99.5% pure.

In the apparatus 10, the bellows 22 is preferably made of Inconel towithstand pressure and high temperature and outer housing 16 ispreferably made of high temperature steel for the same reason. Themaximum compaction force for a 14 inch diameter billet is 800 tons andmaximum pressure for that same size billet during vacuum pressing isalso 800 tons at 10,000 psi. The maximum sintering temperature is 850°F. and the sintering retort vacuum maximum is 400 microns.

Having thus disclosed a preferred embodiment of the present invention,it will be understood that numerous additions and modifications arecontemplated and may occur to those who have the benefit of the teachingherein. Accordingly, the scope hereof is to be limited only by theappended claims and their equivalent.

1. A method of fabricating ingots of ceramic-metal matrix composites,the method comprising the steps of: preparing a blended powder mixtureof a selected ceramic and a selected metal matrix material; compactingsaid mixture; outgassing and sintering said compacted mixture at aselected temperature for a selected period of time vacuum pressing thesintered mixture until a billet is formed, said billet having a minimumdensity of 99% of theoretical maximum density; and cooling said billet;said compacting step comprising the steps of loading said mixture into asealable compression die having a die punch and a valve controlleddegassing outlet; and applying a compression force on said die punchwhile preventing air from entering said mixture.
 2. The method recitedin claim 1 wherein said applying step comprises the step of employing asealed bellows surround an end of said die punch for preventing air fromentering said mixture.
 3. The method recited in claim 1 wherein saidoutgassing step comprises the step of channeling gases from said mixturethrough an degassing outlet of said die, and controlling outgassingthrough said outlet by using said valve on controlling said outlet.
 4. Amethod of fabricating ingots of ceramic-metal matrix composites, themethod comprising the steps of: preparing a blended powder mixture of aselected ceramic and a selected metal matrix material; compacting saidmixture; outgassing and sintering said compacted mixture at a selectedtemperature for a selected period of time; vacuum pressing the sinteredmixture until a billet is formed, said billet having a minimum densityof 99% of theoretical maximum density; and cooling said billet; andwherein said preparing step comprises the step of keeping said ceramicand metal matrix powder mixture at least 99.5% pure.